Electric detonator



Sept. 11, 1956 A. MacLEOD 2,762,302

ELECTRIC DETONATOR Filed Nov. 16, 1951 INVENTOR. NORMAN A. MACLEOD,

BY QK W ATTORNEY United States Patent Ufice Patented Sept. 11, I956 72,762,302 ELECTRIC DETONATOR Norman A. MacLeod, Altadena, CalifApplication November 16, 1951, Serial No. 256,805

' *3 Claims. or. 102 zs (Granted under Title 35, U. s. Code 1952 a...266) 1hisinvention relates to an electric detonator for explosives, andmore particularly to a gap assembly for use in detonators of the typewherein an electrical current bridging a gap initiates an explosionin'adjacent explosive material; V p

' 'It is well'known in the art thatdetonators can be made using twoconductors separated by a gap. The old method of manufacture employs twoinsulated wires which are-twisted-together and the twisted portionembedded in a plastic:matrix. A portion of the matrix is ground off,leaving two exposed wire ends on a plane surface and separated by theinsulation on the wires. The old method has disadvantages since theindividual twisting and matrixing of each pair of wires results in lackof uniformity; the twist is difiicult to control because the wire endstend to spring out of position; it is difficult to matrix a twisted wirepair without forming cavities; the round wires used provide a poor sparkdischarge shape; and it is possible to attain a small gap only by usingwires with a thin separating insulation which is weak mechanically.

In one form of the present invention, a current conducting foil ispositioned between two strips of nonconductive material to form acomposite strip. The material used may conveniently be a plastic such asvinyl or acrylic resins. Two of the composite strips are then joinedwith their foil edges opposed so that the distance between the strips offoil controls the gap distance at the end surface. A convenient methodof manufacturing the gap assembly is to form the composite strips inrather long lengths, then cement two of them side by side with theirfoils in the same plane, or in parallel adjacent planes, and cut thelong assembly into short sections which are of uniform cross sectionthroughout.

It is an object of this invention to provide a simple, sturdy electricdetonator in which optimum gap shapes and gap dimensions can beprecisely reproduced.

Other objects and many of the attendant advantages of the invention willbe readily appreciated as the same becomes better understood byreference to the following description.

The instant invention is illustrated diagrammatically in theaccompanying drawings, in which:

Fig. 1 is a perspective view of a gap assembly embodying the invention;

Fig. 2 is a similar view of a modified form of the invention;

Fig. 3 is an end view of the assembly wherein a plastic strip ispositioned between the two conductors to enlarge the end gap by acontrolled amount;

Fig. 4 is an end view of a gap assembly in which one electrode is metalfoil and the other electrode is a wire;

Fig. 5 is a longitudinal view, partly in section, of a detonatorincluding a further modified form of gap assembly; and

Fig. 6 is a view in perspective of the rear end of a gap assemblyshowing a manner of connecting the same to a firing current.

Referring first to the embodiment shown as Fig. 1, a conductive foil 10is placed between strips of plastic 12 to form a composite strip. In allof the figures of the drawing, the thickness of the foil 10 isexaggerated for clarity, ..but it will be understood that the thicknessis such as is implied in the term foil. Two of such composite strips arethen bonded together by means of insulating cement 13 with theiradjacent foil edges opposed as shown in Fig.1. The long assembly is thencut to convenient lengths toform gap assemblies, and the end surface ofeach gap assembly is then ground or polished'to produce a. uniformsurface. The'gap distance is controll'edby the thickness of the cement.bond 13.

The embodiment shown in-Fig-. 2. is similar tothat shown inFig. 1,except that in Fig. 2 the two composite strips are. displacedlaterallylfrom each other in order to increase the gap between foilstrips 10 where a larger end gap is desired.

In Fig. 3 is shown the use of opposing electrodes10 separated by anon-conducting. film 15, bonded by cement films 13. It Will be seen thatthe gap distance can be accurately controlled by using film of any giventhickness as a spacer. Except .for the presence of a spacer inthe zone,which in the embodiments heretofore described is occupied by cementonly, the structure is identical with that shown at Fig. 1.

Fig. 4 shows an assembly containing foil 10 as one conductor and a wire17, embedded in a plastic strip 27 and having a thickness greater thanthat of the foil, as the other conductor. In this modification, the foilfurnishes the desirable point electrode and the relatively large area ofthe wire enables the gap size to remain nearly constant even though thetwo strips may be slightly out of alignment; a failure to properly aligntwo foil-andplastic strips of the type shown in Fig. 1 may result in anundesired gap distance. The wire may be flattened in cross section asshown, in order that displacement of the plastic strip 27 will notgreatly affect the length of the gap. 7

The detonator shown in Fig. 5 comprises a gap assembly 23 which issimilar to that shown in Fig. 1 except that it has been turned down tocylindrical shape. A cap 16, of known design and of insulating material,is fitted over the gap assembly and contains an initiator explosive 18such as lead azide. If desired, a film 20 of graphite or the like may beapplied to the polished end of the gap assembly. In order to obtain abetter bond between the elements, the foil 10 may be formed withperforations 22, into which the plastic material 12 may be pressed whilehot, as shown. Similar bonding means may be employed in the other formsof the invention shown in the drawing.

Fig. 6 shows the rear end of a gap assembly of the type shown in Fig. 3,and indicates how the electrical current may be supplied thereto to firethe charge. In the example shown, bores 30 are drilled in the respectivecomposite strips in the plane of the foil strips, to receive metal plugs32 which conduct current to the respective foil strips. Alternatively,female connectors may be used with any convenient receptacle to providesturdy reliable connections which can be rapidly made or broken.

The forms of the invention shown in Figs. 1-5 may be provided with anysuitable connectors, such as the plugs 32 shown in Fig. 6. The currentis prevented from traveling from one conductor to the other by thecement bond 13 except at the gap at the end of the assembly. The currentflow across the gap serves to initiate explosion of explosive materialpositioned adjacent to the thin graphite film which, though notnecessary to the operation of the gap assembly, aids in establishing acurrent flow across the gap.

It is possible to use foil and plastic strips of indefinite length andthus prepare many gap assemblies in a rela- I tively few operations. Thestructure resulting when two long composite strips are cemented togetheris divided into a number of duplicate assemblies by a simple cuttingOperation. Since the significant dimensions are established w before thecutting operation .takes place, gap assemblies wise with a fixeddistance between adjacent edges ofsaid conductors controlled by thethickness ofv the cement,

' and forming a plane end surface bounded by onepair of formed by theprocess will have reproducible character-' I istics as detonators.

Obviously'many modifications and variations of. the 10 present inventionare possible in the light of the above teachings. For example, the foil10 of Fig. 1 may be the plastic or by any conductor having the requisiteshape and uniform cross section. As another example, the

' graphite film bridging the electrode gap may be replaced by anysuitable conductive coating'or by a bridge \wire.

' replaced by a metallic film deposited. on the surface of It is,therefore, to be understood that within the scope The inventiondescribed herem may be manufactured and used by or for the Government ofthe United States I of America for governmental purposes without thepay- I ment of any royaltiesthereon or therefor.

.WhatisclaimedisL I j f 1. In a method for the manufacture of detonatorgap assemblies, the steps comprising forming a composite longitudinallyuniform stripincluding an electrical con- 7 ductor bondedbetween stripsof substantially rigid plastic insulating material, cementing two ofsaid units lengthofvth'e appended claims the invention maybe practicedotherwise than as specifically described. 20

conductor ends.

2. The invention defined in claim 1 comprising in addition the step ofcutting the cemented composite strips into a plurality of gap assembliesidentical in cross section and having plane ends.

3. In an electrical detonator, a gap assembly comprising :two compositelongitudinally uniform units each'including two strips of plasticmaterial separated bymetallic foil and terminating in a common plane,and insulating I adhesive securing said units together and maintainingthe adjacent edges of the foil at the ends of; said units opposed toeach other and in predetermined spaced relation.

7 7 References Cited in the file of 'this'patent UNITED STATES PATENTS148,338 'Varney -1. Mar. "10,1874 1,542,825 Newton June 23, 1925 I 71,926,842 Dubilier Sept. 12, 1933 2,437,153 Cohan Mar. 2,, 19482,619,443 Robinson Nov. 25-, 1952 2,654,060 Stovall et a1. Sept. 29, 1953 FOREIGN'PATENTS 7 527,290 :France "a July 21, 1921 969,709 I 7France May I-il, 1950

